The underlying concept behind a dairy operation is that cows must become pregnant, carry a calf to term, and with parturition begin producing milk. Not only is pregnancy a prerequisite for milk production in the dairy herd, but female offspring become the future producers in both the beef and dairy herd. Using current production capabilities with average herd cows, it is believed in the dairy industry that lifetime production of cattle is maximized when calving intervals are maintained at approximately twelve month, approximately 360 day, intervals. Using this 360 day interval, parturition is considered day zero (0). Following parturition, the animal requires a period of time to return to estrus. In dairy cows this can occur as early as the first 14 days, but more frequently occurs later, e.g., 30 to 72 days. Since gestation lengths of cattle are fixed at about 280 days, cows must become pregnant again within about 150 days postpartum so that a reasonable calving interval can be maintained while at the same time maximizing production. The cow being on an approximately 21 day estrus cycle, a delay in the return to estrus, or failure to conceive with insemination following return to estrus, can cause disruption of the 360 day interlactation interval. However, the early postpartum interval represents a period when milk yields are maximum, and appetite (feed intake) is usually inadequate to meet production needs. Cattle must therefore mobilize their own body stores of fat to support milk production. When tiffs mobilization occurs, cattle are said to be in a state of negative energy balance. Negative energy balance is considered to be detrimental to reproductive efficiency in cattle. See, e.g., Butler, et al., J. Animal Science, 53:742 (1981). Thus, it is one goal of the herd manager to maximize milk production while at the same time minimizing the deleterious effect of somatotropin on energy balance.
Bovine somatotropin is produced in the anterior pituitary gland and the metabolic effects of this 190 to 191 amino acid protein are varied. For instance, it has been known for years that this substance causes an increase in milk production. However, this is not the only effect of somatotropin as other metabolic changes, including glucose and lipid metabolism, skeletal growth, and protein synthesis, have been reported. In addition, since high milk production causes the cow to enter negative energy balance, numerous studies have been conducted on the effect of somatotropin on both milk production and reproductive performance. Most of these studies report that somatotropin treatment has no significant effect on reproductive performance; some, using somatotropin at relatively high concentrations or early in the postpartum period, have reported some reduction in reproductive efficiency. These studies which report a negative effect on reproduction have used what can be called a therapeutic dose of somatotropin, i.e. the amount of somatotropin necessary to enhance milk production, which is generally recognized to be about 5 to 50 mg/animal/day.
According to the method of the present invention, it has been found that cattle administered somatotropin starting in the early postpartum period show improved reproductive efficiency compared with herdmates. The dose of somatotropin which is effective at improving reproductive performance is the equivalent of less than 14 mg of somatotropin per cow per day. Treatment at this dose of somatotropin continues until the animal is once again pregnant or until about 100 days postpartum, whichever occurs first. Following cessation of treatment during this first period, treatment with somatotropin continues at a maximally efficacious dose to maintain high milk yields which are typical of animals undergoing somatotropin treatment. Surprisingly, treatment of dairy cattle with somatotropin following the method of the invention also results in an increase in FCM (fat corrected milk) which is greater than that which is seen when only higher doses of somatotropin are used.